BIOL 160: General Biology
Definition Worksheet #7: Chapter 14
Define the following terms related to transciption and translation.
1. Anticodon
2. Codon
3. Elongation stage
4. Frame-Shift mutation
5. Gene
6. Initiation stage
7. mRNA molecule
8. Mutation
9. Promotor region of a gene
10. rRNA molecule
11. RNA polymerase
12. Substitution mutation
13. tRNA molecule
14. Terminator region of a gene
15. Termination stage
16. Transcription
17. Translation
18. Triplet
CHAPTER 14: From DNA to RNA to PROTEIN Molecules:
1. Cells cannot survive unless they are constantly replacing enzymes and other proteins. The instructions for making these molecules are referred to as the “Genetic Code” and are found within the DNA molecules (chromosomes).
2. DNA Replication (covered in chapter 13) only occurs when a cell is actively preparing to divide. This is the only time the DNA is completely unwound and an exact copy is made. For the majority of the time, the DNA strands only unwind in specific areas to expose the genetic code for a specific protein that is needed.
3. The small section of a DNA strand that codes for a specific protein is called a
_, and is the functional unit of heredity. This is why the study of heredity is called genetics.
4. The process of producing a protein molecule from the instructions encoded in the DNA molecules requires two steps:
A. : is the first step, where the DNA message is copied into a complementary code called. This process occurs inside the nucleus as DNA never leaves the nucleus. B. : is the second step that must occur. In this process the RNA molecule leaves the nucleus (after it has been modified slightly) through pores in the nuclear envelope. The RNA makes its way to a ribosome where it directs the assembly of amino acids into a polypeptide (protein) chain. This process therefore occurs in the cytoplasm of the cell.
5. TRANSCRIPTION: Converting DNA into RNA
A. This is a process that converts one nucleic acid into another nucleic acid making a complementary copy. This is like a “medical transcriptionist” converting a “voice message” in say English to a “written message” still in English. The two forms are of the same language, but in a different form.
B. Remember that DNA and RNA are similar and yet different:
1. They are both made of subunits called . (they are both classified as polynucleotides). 2. DNA is a_ strand, RNA is a strand.
3. DNA uses the sugar _ RNA uses the sugar .
4. DNA uses nucleotides A, C, G & _, RNA uses nucleotides A, C, G & _ .
RNA Strand
DNA Template
C. Given the following DNA sequence, what is the complementary RNA transcript that would be produced by transcription of the DNA? TAC AAG ATA ACA TTT GTC ATT
_ _ _ _ _ _ _
(RNA transcript)
D. There are three types of RNA molecules produced in the nucleus that are all formed by transcription of different sections (genes) of the DNA sequence:
1. _ RNA (mRNA): has the genetic instructions for making a specific protein (polypeptide). This is like the “blue-print” for the project that will ultimately make a protein molecule.
2. RNA (rRNA): contains the genetic instructions for making a Ribosome that will ultimately take part in the formation of the new protein molecule. The Ribosome in a sense acts like the factory (or an assembly line in a factory) where the blue-print will be read and assembly of the protein will take place.
3. RNA (tRNA): contains the genetic instructions for making a specific carrier molecule that will attach to a specific amino acid in the cytoplasm and deliver it to the Ribosome for assembly into a protein molecule. The tRNA molecule in a sense acts like the workers in the factory, reading the blue-print.
E. There are several important enzymes needed for Transcription to occur, just like in DNA replication, each with a specific function to perform.
1. : attaches to the helix strands of DNA and break the hydrogen bonds, separating the double strands, forming a little “bubble” in the DNA chain. Unlike DNA replication, only one bubble forms at a specific location on a specific chromosome where the genetic instructions are located for the specific protein needed to be synthesized. Once the “bubble” forms, the enzyme moves down the chain in one direction increasing the size of the “bubble”. This works much like the clasp of a zipper opening it up.
2. : attaches to the DNA and begins to “read” the DNA molecule, forming a strand of RNA. a. Unlike the DNA Polymerase enzyme in DNA replication, this enzyme only reads one strand of the original DNA molecule, not both, and only reads a small section of the DNA strand, not the entire strand. b. Like DNA Polymerase, this enzyme also only “reads” the original DNA strand of nucleotides from the_ end toward the_ end of the original DNA molecule.
c. The new RNA molecule is “growing” from theend to theend. d. Unlike the DNA Polymerase enzyme, this enzyme uses the nucleotide
_ (U) instead of (T) as the complement to Adenine (A) e. Each gene is responsible for regulating its own activity.
F. Transcription can be divided into three distinct stages: 1. INITIATION STAGE: a. At the beginning of each gene there is a region of the DNA called the DNA that signals the RNA Polymerase enzyme where to attach to the DNA strand. b. This stage begins as the enzyme attaches to the DNA strand and begins to “read” the original DNA “message”. c. At the very end of the promoter DNA is a specific set of nucleotides that signals the start of the actual set of instructions. This is called the “start signal”. 2. ELONGATION STAGE: a. This is the process where the _ enzyme continues to “read” the DNA strand, creating the complementary mRNA strand.
b. As the process continues, the RNA strand gets longer and longer as more and more nucleotides are added to the strand.
3. TERMINATION STAGE:
a. At the end of each gene there is a region of the DNA called the TERMINATOR DNA that signals the RNA Polymerase enzyme that the end of the gene (genetic sequence for a particular protein) has been reached and it is now time to detach from the DNA strand.
b. At the very beginning of this section is a specific set of nucleotides that signals the end of the actual set of instructions called the “stop signal” and the RNA Polymerase enzyme stops reading the DNA sequence.
G. Several modifications (finishing touches) are made to the pre-RNA strands before they leave the nucleus.
1. A Cap _ (methyl and phosphate groups) is added to the 5’ end of the RNA strand. These groups will help the final mRNA attach itself to the Ribosome. In a sense, it also protects the beginning of the set of instructions like a hat protects the head.
2. A Poly-A Tail is added to the 3’ end of the RNA strand. This will generally contain anywhere from 100-300 Adenine (A) nucleotides (often called a poly-A tail). This strand of nucleotides will combine with proteins and coil up. The longer the tail and the more coiled it becomes influences how long the mRNA will function in the cytoplasm. If only a small amount of the protein is needed, a short tail will be attached, if much of the protein is needed, a long tail will be attached. 3.Intron _ of the RNA strand are removed (snipped out). These sections of a gene are not needed for making a specific protein. By removing these sections, different cells can use the same gene to make 10 or more different proteins. Think of these as sections that are “in the way” or need to stay “inside the nucleus”.
4. Exon of the RNA strand are sliced together (reconnected) to form the final version of the mRNA. Once these are reconnected the mRNA is ready to leave the nucleus. Think of these as sections that need to be “exported” from the nucleus as part of the final message.
6. TRANSLATION: Deciphering the mRNA genetic code into a protein molecule.
A. The connection between genes and proteins is found in the protein-building “words” found in the mRNA molecules.
B. In the English language “words” can be of any length, however the genetic code
on the mRNA strand is always “read” by the Ribosomes 3nucleotides at a time. This is because the DNA nucleotides (four of them A,T,C,G) must code for 20 different amino acids. 1. If it was a 1:1 ratio, there could only be 4 amino acids (41=4). 2. If two nucleotides were used in any order, there could be 16 different amino acids (42=16), still not enough. 3. If three nucleotides were used in any order, there could be 64 different amino acids (43=64). Three is all that is needed to give more than enough information, since there are only 20 different amino acids. 4. A group of three nucleotides on a DNA strand is called a TRIPLET. 5. A complementary group of three nucleotides on a mRNA strand is called a
_codon. In other words, “the secret ‘CODE is ON’ the mRNA strand”, or described another way, “the DNA’s ‘message’ on the mRNA is in a secret ‘code’.
6. To understand a “secret code” one needs a “code book”. For protein synthesis, the code book is for the CODONS on the mRNA, not for the DNA triplets.
7. As mentioned, there are 64 different
“3-letter” combinations possible.
8. Each 3 letter (nucleotide) sequence
codes for one particular amino acid.
9. The table at the right shows the
different combinations possible
and the amino acids for which
each codes.
a. Some combinations code for
the same amino acid (there are
64 possibilities, but only need
to make 20 amino acids, so no
need to let them go to waste).
b. One codon signals the START
of the message. It is AUG_. c. There are three codons that signal the END of the message. They are:UAA_, UAG_, andUGA_.
C. Given the following mRNA sequence that is complementary to a section of the original DNA molecule, use the table of mRNA Codons from the textbook or on the previous page to determine the amino acid sequence of the polypeptide chain. AUG UUC UAU UGU AAA CAG UAA Met Phe Tyr Cys Lys Gln Stop
_ _ _ _ _ _ _
(Amino acid sequence)
D. The tRNA molecule also has a group of 3 nucleotides that is complementary to
the CODON on the mRNA, and is called an anticodon_. E. The CODON on the mRNA is complementary to the TRIPLET on the original DNA, and is also complementary to the ANTICODON on the tRNA; therefore the ANTICODON is essentially the same genetic code as the original DNA. This is how the tRNA molecule knows which amino acid to transport to the Ribosome. F. The tRNA molecules attach to special enzymes in the cytoplasm which helps them link to specific amino acids. The tRNA then transports the specific amino acid to the rRNA (Ribosome) where it joins to other amino acids by a peptide bond, ultimately forming a polypeptide (protein) molecule. G. Translation can also be divided into three distinct stages: 1.InitiationSTAGE: this gets everything ready to begin. a. to begin the process of converting the mRNA message into a protein chain, the mRNA binds to a ribosome .
b. the ribosome begins to move along the mRNA strand “looking” for a
specific mRNA codon called the starter _ codon (AUG). c. an initiator_ tRNA molecule arrives at the Ribosome and the anti-codon on the t-RNA forms hydrogen bonds with the “start” codon on the m-RNA strand.
2. _Elongation STAGE: this is the work in progress.
a. a second t-RNA molecule brings its amino acid to the ribosome and hydrogen bonds form between the anti-codon (on the tRNA) and the codon (on the mRNA).
b. while the two amino acids are held closely together, a peptide bond forms between the two amino acids
c. once this very strong bond forms, the initiator t-RNA separates from its amino acid AND the ribosome and leaves to go “find” another amino acid in the cytoplasm.
d. the ribosome moves down the mRNA strand and the process continues.
e. as the process continues the protein chain gets longer and longer by linking together more and more amino acids.
3. _Termination STAGE: this ends the process of translation.
a. the elongation stage continues until the ribosome reaches a specific codon
called the _stop codon. Since there is no tRNA that recognizes this message, the process stops.
b. Special proteins known as FACTORS trigger the mRNA and protein to break away from the Ribosome. For a typical protein molecule, this entire process of translation takes about 20 seconds. The mRNA strand remains intact and may come in contact with another ribosome and begin translation before the first one is even finished. In this way, many proteins can be made quickly. Eventually, the mRNA breaks down and is recycled to make new DNA or RNA strands in the nucleus. 7. Any change in the DNA sequence that alters the genetic code and may change the protein being produced is called amutation_. If the protein is essential for metabolism or cell structure, the cell may also be altered, and in some cases become cancerous.
A. There are two main types of mutations that occur in the cell:
1. Substitution MUTATIONS: these occur when one nucleotide is wrongly paired with another nucleotide. The effects may or may not change the desired amino acid in the final protein (remember more than one codon can signal for some amino acids) and therefore will not be evident or if it does change the code, the effects are generally less sever than the other types of mutations because only one codon and therefore only one amino acid is affected.
2. FrameshiftMUTATIONS: occur when an additional nucleotide is added to the sequence (insertion_) or
one nucleotide is removed from the sequence (deletion_). The effects of this type of mutation are very severe because many codons are affected.
B. There are a number of known factors that may cause mutations:
1. Ionizing Radiation_: most commonly caused by x-rays.
2. _Non-Ionizing Radiation : most commonly caused by ultraviolet rays from the sun (or tanning beds). 3._Alkylating AGENTS: natural and synthetic chemicals that attach to the nitrogenous bases or phosphate groups in DNA.
C. Mutations change the meaning of the coded genetic message. Using the genetic code chart, translate the following mRNAs into amino acid sequences and answer the questions that follow. 1. Original mRNA nucleotide sequence:
a. What would be the correct amino acid sequence formed from this mRNA?
2. Base Substitution Mutation (*) in the original mRNA: Only one nucleotide is changed in this mutation (labeled with the *), where the letter ‘A’ in the above sequence was replaced with the letter ‘C’. *
a. What is the new amino acid sequence formed from this mRNA? cau b. How many amino acids remained the same after the mutation occurred? 6 c. How many changed as a result of this mutation? 1 d. Why are all of these types of mutations not necessarily harmful? Sometimes alternating codons specify the same amino acid. 3. Frame Shift (insertion) Mutation (*) in the original mRNA: Only one nucleotide was changed in this mutation (labeled with the *), where the letter ‘G’ was inserted before the letter ‘A’. *
a. What is the new amino acid sequence produced from this mRNA? METPROASPGLUTYRSTOPVAL b. How many amino acids remained the same as a result of this mutation? 3 4. Which mutation had the greatest effect on the protein chain and why? The framshift mutation. It altered more amino acids. 8. REVIEW the “Key Concepts” to know about Protein Synthesis: A. Converting DNA into RNA and finally into a Protein chain takes place in several steps: 1.Transcription: takes place in the nucleus and converts the DNA into RNA. It involves 3 stages: a.Initiation : RNA Polymerase attaches to one section of a DNA strand and begins to “read” the DNA.
b. Elongation: RNA Polymerase continues to “read” the DNA strand creating a longer and longer RNA molecule. c.Termination : RNA Polymerase stops “reading” the DNA and releases the RNA molecule.
2. Modification_ of the RNA: before the molecule can leave the nucleus, several changes need to be made.
a. A _Cap is added to the 5’ end
b. A A-Tail_ is added to the 3’ end
c. Intron_ are removed from the RNA molecule d. Exons_ are reconnect (spliced together) to form the finished mRNA molecule
3. Translation: takes place in the cytoplasm (specifically at a ribosome) and converts RNA into Protein. It also involves three stages:
a. Initiation _: mRNA attaches to the ribosome and the initiator t-RNA attaches to the mRNA.
b. Elongation_: additional tRNA molecules deliver specific amino acids to the ribosome, allowing them to form strong peptide bonds to other amino acids, and the new protein chain grows in length.
c. Termination_: a stop codon is reached on the mRNA and release factors allow the mRNA and protein to break free from the ribosome.
B. There are three different types of RNA with very different functions:
1. MessengerRNA (mRNA): carries the secret CODE out of the nucleus and delivers it to the ribosomes 2.Transfere_ RNA (tRNA): finds the correct amino acid in the cytoplasm that is required and delivers it to the ribosome, matching its anticodon to the codon on the mRNA 3. _Ribosomal RNA (rRNA): the organelle where the actual protein is assembled
9. This chapter summarizes the key steps in the flow of genetic information from DNA to protein. We will fill this diagram in together in class as a final review.
Sample Test Questions for Chapter 14:**
1. Where in the cell does transcription take place? a. nucleus
b. cell membrane
c. ribosome
d. chromosome
e. mitochondria
2. When RNA is made, the RNA base always pairs with the base __ in the original DNA molecule.
a. U……T
b. T……G c. U……A
d. A……U
e. T……A
3. The sequence of three nucleotide nitrogenous bases can specify the identity of
a. a specific gene b. a single DNA molecule
c. a single protein molecule
b. a single amino acid molecule
c. all of the above
4. The transfer of genetic information from mRNA to a polypeptide is called a. translation
b. transcription
c. initiation
d. elongation
e. replication
5. At one point as a cell carried out its regular functions, the nucleotide CTA was paired with the nucleotide GAT. This pairing occured a. in a double-stranded DNA molecule
b. during translation
c. when an mRNA codon paired with a tRNA anticodon
d. it is impossible to say with the information given
6. If the nucleotide sequence of a section of DNA strand is “GTAACT”, which of the following choices is the correct nucleotide sequence found on the corresponding strand of mRNA copied?
a. CATTGA
b. GUAACU
c. GTAACT d. CAUUGA
e. GAUUCA
7. Which of the following enzymes catalyzes the linking together of RNA nucleotides to form an mRNA molecule?
a. RNA ligase b. RNA polymerase
c. ribosome
d. reverse transcriptase
e. tRNA enzyme
8. When DNA replication occurs, DNA polymerase always reads the template strand of the original DNA molecule in which direction? a. From the 3’ to the 5’
b. From the 5’ to the 3’
c. one strand of DNA reads in one direction (3’ to 5’) while the other strand of DNA reads in the other direction (5’ to 3’)
9. A gene is
a. the same thing as a chromosome b. the information for making a polypeptide
c. made of RNA
d. made by a ribosome
e. made of protein
10. A messenger RNA molecule for making a protein is made in the nucleus and sent out to a ribosome. The ribosome reads the mRNA message and makes a protein containing 40 amino acids. The mRNA strand consists of at least how many nucleotides?
a. 30
b. 40 c. 120
d. 360
11. True or False: The nitrogenous bases that pair together to connect the two parallel strands forming DNA (or in other words the “rungs” of the DNA ladder) are held together by covalent bonds.
12. Which of the following changes occur before eukaryotic mRNA leaves the nucleus?
a. a nucleotide cap is added to the strand
b. the molecule is spliced
c. a long nucleotide tail is added to the strand d. all of the above
e. only A and C are true statements
Definition Worksheet #7: Chapter 14
Define the following terms related to transciption and translation.
1. Anticodon
2. Codon
3. Elongation stage
4. Frame-Shift mutation
5. Gene
6. Initiation stage
7. mRNA molecule
8. Mutation
9. Promotor region of a gene
10. rRNA molecule
11. RNA polymerase
12. Substitution mutation
13. tRNA molecule
14. Terminator region of a gene
15. Termination stage
16. Transcription
17. Translation
18. Triplet
CHAPTER 14: From DNA to RNA to PROTEIN Molecules:
1. Cells cannot survive unless they are constantly replacing enzymes and other proteins. The instructions for making these molecules are referred to as the “Genetic Code” and are found within the DNA molecules (chromosomes).
2. DNA Replication (covered in chapter 13) only occurs when a cell is actively preparing to divide. This is the only time the DNA is completely unwound and an exact copy is made. For the majority of the time, the DNA strands only unwind in specific areas to expose the genetic code for a specific protein that is needed.
3. The small section of a DNA strand that codes for a specific protein is called a
_, and is the functional unit of heredity. This is why the study of heredity is called genetics.
4. The process of producing a protein molecule from the instructions encoded in the DNA molecules requires two steps:
A. : is the first step, where the DNA message is
copied into a complementary code called . This process occurs inside the nucleus as DNA never leaves the nucleus.
B. : is the second step that must occur. In this process the RNA molecule leaves the nucleus (after it has been modified slightly) through pores in the nuclear envelope. The RNA makes its way to a ribosome where it directs the assembly of amino acids into a polypeptide (protein) chain. This process therefore occurs in the cytoplasm of the cell.
5. TRANSCRIPTION: Converting DNA into RNA
A. This is a process that converts one nucleic acid into another nucleic acid making a complementary copy. This is like a “medical transcriptionist” converting a “voice message” in say English to a “written message” still in English. The two forms are of the same language, but in a different form.
B. Remember that DNA and RNA are similar and yet different:
1. They are both made of subunits called . (they are both classified as polynucleotides).
2. DNA is a _ strand, RNA is a strand.
3. DNA uses the sugar _ RNA uses the sugar .
4. DNA uses nucleotides A, C, G & _, RNA uses nucleotides A, C, G & _ .
RNA Strand
DNA Template
C. Given the following DNA sequence, what is the complementary RNA transcript that would be produced by transcription of the DNA?
TAC AAG ATA ACA TTT GTC ATT
_ _ _ _ _ _ _
(RNA transcript)
D. There are three types of RNA molecules produced in the nucleus that are all formed by transcription of different sections (genes) of the DNA sequence:
1. _ RNA (mRNA): has the genetic instructions for making a specific protein (polypeptide). This is like the “blue-print” for the project that will ultimately make a protein molecule.
2. RNA (rRNA): contains the genetic instructions for making a Ribosome that will ultimately take part in the formation of the new protein molecule. The Ribosome in a sense acts like the factory (or an assembly line in a factory) where the blue-print will be read and assembly of the protein will take place.
3. RNA (tRNA): contains the genetic instructions for making a specific carrier molecule that will attach to a specific amino acid in the cytoplasm and deliver it to the Ribosome for assembly into a protein molecule. The tRNA molecule in a sense acts like the workers in the factory, reading the blue-print.
E. There are several important enzymes needed for Transcription to occur, just like in DNA replication, each with a specific function to perform.
1. : attaches to the helix strands of DNA and break the hydrogen bonds, separating the double strands, forming a little “bubble” in the DNA chain. Unlike DNA replication, only one bubble forms at a specific location on a specific chromosome where the genetic instructions are located for the specific protein needed to be synthesized. Once the “bubble” forms, the enzyme moves down the chain in one direction increasing the size of the “bubble”. This works much like the clasp of a zipper opening it up.
2. : attaches to the DNA and begins to “read” the DNA molecule, forming a strand of RNA.
a. Unlike the DNA Polymerase enzyme in DNA replication, this enzyme only reads one strand of the original DNA molecule, not both, and only reads a small section of the DNA strand, not the entire strand.
b. Like DNA Polymerase, this enzyme also only “reads” the original DNA
strand of nucleotides from the _ end toward the _ end of the original DNA molecule.
c. The new RNA molecule is “growing” from the end to the end.
d. Unlike the DNA Polymerase enzyme, this enzyme uses the nucleotide
_ (U) instead of (T) as the complement to Adenine (A)
e. Each gene is responsible for regulating its own activity.
F. Transcription can be divided into three distinct stages:
1. INITIATION STAGE:
a. At the beginning of each gene there is a region of the DNA called the
DNA that signals the RNA Polymerase enzyme where to attach to the DNA strand.
b. This stage begins as the enzyme attaches to the DNA strand and begins to “read” the original DNA “message”.
c. At the very end of the promoter DNA is a specific set of nucleotides that signals the start of the actual set of instructions. This is called the “start signal”.
2. ELONGATION STAGE:
a. This is the process where the _ enzyme continues to “read” the DNA strand, creating the complementary mRNA strand.
b. As the process continues, the RNA strand gets longer and longer as more and more nucleotides are added to the strand.
3. TERMINATION STAGE:
a. At the end of each gene there is a region of the DNA called the TERMINATOR DNA that signals the RNA Polymerase enzyme that the end of the gene (genetic sequence for a particular protein) has been reached and it is now time to detach from the DNA strand.
b. At the very beginning of this section is a specific set of nucleotides that signals the end of the actual set of instructions called the “stop signal” and the RNA Polymerase enzyme stops reading the DNA sequence.
G. Several modifications (finishing touches) are made to the pre-RNA strands before they leave the nucleus.
1. A Cap _ (methyl and phosphate groups) is added to the 5’ end of the RNA strand. These groups will help the final mRNA attach itself to the Ribosome. In a sense, it also protects the beginning of the set of instructions like a hat protects the head.
2. A Poly-A Tail is added to the 3’ end of the RNA strand. This will generally contain anywhere from 100-300 Adenine (A) nucleotides (often called a poly-A tail). This strand of nucleotides will combine with proteins and coil up. The longer the tail and the more coiled it becomes influences how long the mRNA will function in the cytoplasm. If only a small amount of the protein is needed, a short tail will be attached, if much of the protein is needed, a long tail will be attached.
3.Intron _ of the RNA strand are removed (snipped out). These sections of a gene are not needed for making a specific protein. By removing these sections, different cells can use the same gene to make 10 or more different proteins. Think of these as sections that are “in the way” or need to stay “inside the nucleus”.
4. Exon of the RNA strand are sliced together (reconnected) to form the final version of the mRNA. Once these are reconnected the mRNA is ready to leave the nucleus. Think of these as sections that need to be “exported” from the nucleus as part of the final message.
A. The connection between genes and proteins is found in the protein-building “words” found in the mRNA molecules.
B. In the English language “words” can be of any length, however the genetic code
on the mRNA strand is always “read” by the Ribosomes 3 nucleotides at a time. This is because the DNA nucleotides (four of them A,T,C,G) must code for 20 different amino acids.
1. If it was a 1:1 ratio, there could only be 4 amino acids (41=4).
2. If two nucleotides were used in any order, there could be 16 different amino acids (42=16), still not enough.
3. If three nucleotides were used in any order, there could be 64 different amino acids (43=64). Three is all that is needed to give more than enough information, since there are only 20 different amino acids.
4. A group of three nucleotides on a DNA strand is called a TRIPLET.
5. A complementary group of three nucleotides on a mRNA strand is called a
_codon. In other words, “the secret ‘CODE is ON’ the mRNA strand”, or described another way, “the DNA’s ‘message’ on the mRNA is in a secret ‘code’.
6. To understand a “secret code” one needs a “code book”. For protein synthesis, the code book is for the CODONS on the mRNA, not for the DNA triplets.
7. As mentioned, there are 64 different
“3-letter” combinations possible.
8. Each 3 letter (nucleotide) sequence
codes for one particular amino acid.
9. The table at the right shows the
different combinations possible
and the amino acids for which
each codes.
a. Some combinations code for
the same amino acid (there are
64 possibilities, but only need
to make 20 amino acids, so no
need to let them go to waste).
b. One codon signals the START
of the message. It is AUG_.
c. There are three codons that
signal the END of the message.
They are: UAA_, UAG_,
and UGA_.
C. Given the following mRNA sequence that is complementary to a section of the original DNA molecule, use the table of mRNA Codons from the textbook or on the previous page to determine the amino acid sequence of the polypeptide chain.
AUG UUC UAU UGU AAA CAG UAA
Met Phe Tyr Cys Lys Gln Stop
_ _ _ _ _ _ _
(Amino acid sequence)
D. The tRNA molecule also has a group of 3 nucleotides that is complementary to
the CODON on the mRNA, and is called an anticodon_.
E. The CODON on the mRNA is complementary to the TRIPLET on the original DNA, and is also complementary to the ANTICODON on the tRNA; therefore the ANTICODON is essentially the same genetic code as the original DNA. This is how the tRNA molecule knows which amino acid to transport to the Ribosome.
F. The tRNA molecules attach to special enzymes in the cytoplasm which helps them link to specific amino acids. The tRNA then transports the specific amino acid to the rRNA (Ribosome) where it joins to other amino acids by a peptide bond, ultimately forming a polypeptide (protein) molecule.
G. Translation can also be divided into three distinct stages:
1. Initiation STAGE: this gets everything ready to begin.
a. to begin the process of converting the mRNA message into a protein
chain, the mRNA binds to a ribosome .
b. the ribosome begins to move along the mRNA strand “looking” for a
specific mRNA codon called the starter _ codon (AUG).
c. an initiator_ tRNA molecule arrives at the Ribosome and the anti-codon on the t-RNA forms hydrogen bonds with the “start” codon on the m-RNA strand.
2. _Elongation STAGE: this is the work in progress.
a. a second t-RNA molecule brings its amino acid to the ribosome and hydrogen bonds form between the anti-codon (on the tRNA) and the codon (on the mRNA).
b. while the two amino acids are held closely together, a peptide bond forms between the two amino acids
c. once this very strong bond forms, the initiator t-RNA separates from its amino acid AND the ribosome and leaves to go “find” another amino acid in the cytoplasm.
d. the ribosome moves down the mRNA strand and the process continues.
e. as the process continues the protein chain gets longer and longer by linking together more and more amino acids.
3. _Termination STAGE: this ends the process of translation.
a. the elongation stage continues until the ribosome reaches a specific codon
called the _stop codon. Since there is no tRNA that recognizes this message, the process stops.
b. Special proteins known as FACTORS trigger the mRNA and protein to break away from the Ribosome.
For a typical protein molecule, this entire process of translation takes about 20 seconds. The mRNA strand remains intact and may come in contact with another ribosome and begin translation before the first one is even finished. In this way, many proteins can be made quickly. Eventually, the mRNA breaks down and is recycled to make new DNA or RNA strands in the nucleus.
7. Any change in the DNA sequence that alters the genetic code and may change the
protein being produced is called a mutation_. If the protein is essential for metabolism or cell structure, the cell may also be altered, and in some cases become cancerous.
A. There are two main types of mutations that occur in the cell:
1. Substitution MUTATIONS: these occur when one nucleotide is wrongly paired with another nucleotide. The effects may or may not change the desired amino acid in the final protein (remember more than one codon can signal for some amino acids) and therefore will not be evident or if it does change the code, the effects are generally less sever than the other types of mutations because only one codon and therefore only one amino acid is affected.
2. Frameshift MUTATIONS: occur when an
additional nucleotide is added to the sequence (insertion_) or
one nucleotide is removed from the sequence (deletion_). The effects of this type of mutation are very severe because many codons are affected.
B. There are a number of known factors that may cause mutations:
1. Ionizing Radiation_: most commonly caused by x-rays.
2. _Non-Ionizing Radiation : most commonly caused by ultraviolet rays from the sun (or tanning beds).
3. _Alkylating AGENTS: natural and synthetic chemicals that attach to the nitrogenous bases or phosphate groups in DNA.
C. Mutations change the meaning of the coded genetic message. Using the genetic code chart, translate the following mRNAs into amino acid sequences and answer the questions that follow.
1. Original mRNA nucleotide sequence:
a. What would be the correct amino acid sequence formed from this mRNA?
Methionine(start), Proline, Aspartic, Asparagine, Isoleucine, Lysine, Stop
2. Base Substitution Mutation (*) in the original mRNA:
Only one nucleotide is changed in this mutation (labeled with the *), where the letter ‘A’ in the above sequence was replaced with the letter ‘C’.
a. What is the new amino acid sequence formed from this mRNA?
cau
b. How many amino acids remained the same after the mutation occurred?
6
c. How many changed as a result of this mutation?
1
d. Why are all of these types of mutations not necessarily harmful?
Sometimes alternating codons specify the same amino acid.
3. Frame Shift (insertion) Mutation (*) in the original mRNA:
Only one nucleotide was changed in this mutation (labeled with the *), where the letter ‘G’ was inserted before the letter ‘A’.
a. What is the new amino acid sequence produced from this mRNA?
METPROASPGLUTYRSTOPVAL
b. How many amino acids remained the same as a result of this mutation?
3
4. Which mutation had the greatest effect on the protein chain and why?
The framshift mutation.
It altered more amino acids.
8. REVIEW the “Key Concepts” to know about Protein Synthesis:
A. Converting DNA into RNA and finally into a Protein chain takes place in several steps:
1. Transcription: takes place in the nucleus and converts the DNA into RNA. It involves 3 stages:
a.Initiation : RNA Polymerase attaches to one section of a DNA strand and begins to “read” the DNA.
b. Elongation: RNA Polymerase continues to “read” the DNA strand creating a longer and longer RNA molecule.
c.Termination : RNA Polymerase stops “reading” the DNA and releases the RNA molecule.
2. Modification_ of the RNA: before the molecule can leave the nucleus, several changes need to be made.
a. A _Cap is added to the 5’ end
b. A A-Tail_ is added to the 3’ end
c. Intron_ are removed from the RNA molecule
d. Exons_ are reconnect (spliced together) to form the finished mRNA molecule
3. Translation: takes place in the cytoplasm (specifically at a ribosome) and converts RNA into Protein. It also involves three stages:
a. Initiation _: mRNA attaches to the ribosome and the initiator t-RNA attaches to the mRNA.
b. Elongation_: additional tRNA molecules deliver specific amino acids to the ribosome, allowing them to form strong peptide bonds to other amino acids, and the new protein chain grows in length.
c. Termination_: a stop codon is reached on the mRNA and release factors allow the mRNA and protein to break free from the ribosome.
B. There are three different types of RNA with very different functions:
1. Messenger RNA (mRNA): carries the secret CODE out of the nucleus and delivers it to the ribosomes
2. Transfere_ RNA (tRNA): finds the correct amino acid in the cytoplasm that is required and delivers it to the ribosome, matching its anticodon to the codon on the mRNA
3. _Ribosomal RNA (rRNA): the organelle where the actual protein is assembled
9. This chapter summarizes the key steps in the flow of genetic information from DNA to protein. We will fill this diagram in together in class as a final review.
Sample Test Questions for Chapter 14:**
1. Where in the cell does transcription take place?
a. nucleus
b. cell membrane
c. ribosome
d. chromosome
e. mitochondria
2. When RNA is made, the RNA base always pairs with the base __ in the original DNA molecule.
a. U……T
b. T……G
c. U……A
d. A……U
e. T……A
3. The sequence of three nucleotide nitrogenous bases can specify the identity of
a. a specific gene
b. a single DNA molecule
c. a single protein molecule
b. a single amino acid molecule
c. all of the above
4. The transfer of genetic information from mRNA to a polypeptide is called
a. translation
b. transcription
c. initiation
d. elongation
e. replication
5. At one point as a cell carried out its regular functions, the nucleotide CTA was paired with the nucleotide GAT. This pairing occured
a. in a double-stranded DNA molecule
b. during translation
c. when an mRNA codon paired with a tRNA anticodon
d. it is impossible to say with the information given
6. If the nucleotide sequence of a section of DNA strand is “GTAACT”, which of the following choices is the correct nucleotide sequence found on the corresponding strand of mRNA copied?
a. CATTGA
b. GUAACU
c. GTAACT
d. CAUUGA
e. GAUUCA
7. Which of the following enzymes catalyzes the linking together of RNA nucleotides to form an mRNA molecule?
a. RNA ligase
b. RNA polymerase
c. ribosome
d. reverse transcriptase
e. tRNA enzyme
8. When DNA replication occurs, DNA polymerase always reads the template strand of the original DNA molecule in which direction?
a. From the 3’ to the 5’
b. From the 5’ to the 3’
c. one strand of DNA reads in one direction (3’ to 5’) while the other strand of DNA reads in the other direction (5’ to 3’)
9. A gene is
a. the same thing as a chromosome
b. the information for making a polypeptide
c. made of RNA
d. made by a ribosome
e. made of protein
10. A messenger RNA molecule for making a protein is made in the nucleus and sent out to a ribosome. The ribosome reads the mRNA message and makes a protein containing 40 amino acids. The mRNA strand consists of at least how many nucleotides?
a. 30
b. 40
c. 120
d. 360
11. True or False: The nitrogenous bases that pair together to connect the two parallel strands forming DNA (or in other words the “rungs” of the DNA ladder) are held together by covalent bonds.
12. Which of the following changes occur before eukaryotic mRNA leaves the nucleus?
a. a nucleotide cap is added to the strand
b. the molecule is spliced
c. a long nucleotide tail is added to the strand
d. all of the above
e. only A and C are true statements